In apparatus for automatically testing electronic circuits (e.g., analog VLSI devices), test patterns are stored in memory and used to provide input test signals applied to nodes of a circuit under test ("CUT"), and the resulting outputs from the CUT are compared with expected outputs. Test apparatus can also include such analog instruments as high frequency and low frequency signal generators to provide analog test signals to the CUT and high and low frequency digitizers and associated capture memory to digitize and store results.
In order to simultaneously provide control signals for the test apparatus and deliver a large number of bits of data to the CUT synchronously with the clocking of the CUT at its high rate, a high speed sequence controller is used to provide the control bits (also referred to as "microcode" herein) and data bits in parallel at high speed. (The microcode plus data bits at a given clock period are referred to as a "vector".) The control bits are included in a random access memory ("RAM") in the sequence controller, for example, 16K deep to provide 16K instruction steps that can be randomly accessed upon providing the appropriate address to the RAM on an address bus controlled by an address generator in the sequence controller. The test patterns are located in RAMs distributed across separate boards and accessed by the same address bus, often referred to as a "state bus".
In test apparatus providing and detecting mixed digital and analog signals, analog instruments have been controlled by bits of the sequence controller RAM assigned to event lines (for example, 5 lines for 5 bits of information) that were used by the analog instruments to provide a limited set of instructions (i.e., 32 total instructions for all instruments) to these instruments during high speed operation of the sequence controller. The majority of changes in the operation of the analog instruments were thus made by the primary control test computer between high speed tests of the CUT.